(1) Field Of The Invention
This invention relates to the suitable frequency converter for the receiver of a direct conversion system (Zero and IF system) or a LOW-IF system especially with respect to the frequency converter used for the receiver of cable communication or radio communication.
(2) Description Of The Related Art
The circuit shown in FIG. 16 is known as a frequency converter of the receiver of the conventional direct conversion system or a LOW-IF system. In this frequency converter, the modulated radio-frequency signal is inputted from the terminal RF of a mixer circuit 1, and is outputted as signal current from the collector terminal of a transistor Tr1. On the one hand, a local oscillation signal is inputted into a terminal LO, and it changes transistors Tr2 and Tr3 by turns into a conducting state and a cutting state.
For this reason, local oscillation frequency can distribute by turns the signal outputted from the collector of a transistor Tr1 to the collector of transistors Tr2 and Tr3, and the multiplication of a radio-frequency signal and a local oscillation signal is performed.
Consequently, frequency-converted output current is outputted from the collector of transistors Tr2 and Tr3, and is transformed into voltage in the load circuit 3. This voltage is amplified in a buffer amplifier 2, and is outputted as output voltage having small output impedance. In a radio receiver, the output signal of a frequency converter is inputted into a channel-select filter.
In many cases, since it is premised on an input signal being inputted from the source of a voltage signal, the active filter made as an integrated circuit needs the buffer amplifier (output buffer) for the output terminal of a frequency converter. Therefore, a frequency converter consists of a mixer circuit and a buffer amplifier, and a channel-select filter is connected to this buffer-amplifier output terminal.
The problem of the output buffer of a frequency converter is described by B. Razavi""s et al. xe2x80x9cDesign Consideration for Direct Conversion Receiversxe2x80x9d (IEEE Transaction on Circuit and Systems-II:Analog and Digital Signal Processing, Vol. 44, No. 6, June 1997). Moreover, when the output of an output buffer is inputted into a channel-select filter, there are some conditions with which the buffer amplifier of a frequency converter should fill a filter to make it function as a design.
The following four points are mentioned as a difficult thing among those conditions or trade-offs in the conventional circuit.
Namely, 1) The desired signal in the output terminal of a mixer circuit is small value. On the one hand, the signal of an adjacent channel is sometimes large by 60 dB, for example, compared with a desired signal. This is 1000 times on voltage. In order to make noise of an output buffer smaller than a desired signal and not to make a desired signal frequency band generate intermodulations in an output buffer, linear amplification of the desired signal needs to be carried out, even if there is an input of a large adjacent signal.
2) The cut-off frequency of a buffer amplifier needs to be high enough compared with the cut-off frequency of a channel-select filter so that the cut-off frequency of a buffer amplifier may not affect the cut-off frequency of a channel-select filter. Moreover, it is required to amplify linearly at least to the signal of the cut-off frequency of an output buffer, and current consumption of a buffer amplifier needs to be increased.
3) The gain of a mixer circuit is proportional to the parallel impedance of a load circuit and the input impedance of an output buffer. Therefore, in order to make a gain as high as possible, the large input impedance of an output buffer is necessary.
4) The output impedance of an output buffer needs to be small enough compared with the input impedance of a channel-select filter in order not to affect the gain change of the passband of a channel-select filter.
In order not to affect a channel-select filter as above-mentioned, as for the output buffer of the conventional frequency converter, it was requested the broadband characteristic. For this reason, since the signal of an adjacent channel was amplified like the desired signal, it was requested to amplify linearly the signal of a level far larger than a desired signal, too.
On the one hand, a channel-select filter is realizable as an integrated circuit in a direct conversion system or a LOW-IF system. The active filter using the operation amplification circuit etc. is used for the channel-select filter in an integrated circuit.
The examples of these active filters are disclosed in xe2x80x9c1.9 GHz Si Direct Conversion Receiver IC for QPSK Modulation Systemsxe2x80x9d (IEICE Transaction elctronics, Vol.E79-C, No. 5 May 1996) by Takahashi etc. and xe2x80x9cA CMOS Channel-Select Filter for a Direct Conversion Wireless Receiverxe2x80x9d (IEEE Journal of Solid-State Circuits, Vol. 32, No. 5, pp. 722-729, May 1997) by P. J. Chang, A. Rofougaran, and Asad A. Abidi.
The filter which consists of two or more blocks is used for these channel-select filters, and each block cannot remove unnecessary signals of adjacent channels, but the desired characteristic is realized on the whole. Moreover, these blocks also have the function which amplifies a desired signal. That is, amplification of a desired signal and removal of unnecessary signals are performed step by step.
For example, if you amplify 10 times, without removing an adjacent signal when an adjacent signal 1000 times the size of a desired signal is inputted, suppose that an adjacent signal will exceed power-supply voltage. Also in this case, you can get the signal less exceeding power-supply voltage by amplifying a desired signal 10 times and amplifying adjacent signals 0.1 times by changing the characteristic of a filter.
Although the method of amplifying a desired signal is also after removing an unnecessary signal, since the active filter excellent in the frequency selection characteristic does not have the good noise characteristic, a signal-to-noise ratio (SNR) deteriorates by this method.
The step which repeats removing a small quantity of unnecessary signals using the filter of the gently-sloping frequency characteristic, and amplifying a small quantity of desired signals is taken for this reason, often.
The frequency characteristic and the gain of each block are determined from the balance with noise so that the signal which passed two or more blocks may have desired amplitude and the desired frequency characteristic.
Thus, it was asked for the output buffer of a frequency converter having a large signal passband compared with a channel-select filter in order not to affect the characteristic of the designed filter. Therefore, the signal of an unnecessary adjacent channel was also amplified and outputted.
The purpose of this invention is to offer the frequency converter which can obtain the desired receiving characteristic with few current consumption.
Other purpose of this invention is to offer the frequency converter which gives a part of function of a channel-select filter to an output buffer, in order to cancel the above-mentioned conditions 1 and 2.
In order to ease or solve the above-mentioned conditions 3 and 4, other purpose of this invention is to offer the frequency converter which has an output buffer with large input impedance and small output impedance.
According to the fundamental feature of this invention, a frequency converter equipped with the mixer circuit which inputs the radio-frequency input signal and the local oscillation signal which were modulated for communication of information, and changes frequency, and the buffer amplifier which is higher than a desired signal frequency band, and has the low-pass passage characteristic of cut-off frequency lower than adjacent channel carrier frequency is offered.
The frequency converter of this invention is equipped with the buffer amplifier which has the frequency characteristic of the input part of a channel-select filter, and the function as a buffer. Since a desired signal needs to be amplified, the cut-off frequency of this output buffer needs to be higher than a desired signal frequency band. Moreover, in order to remove and amplify the signal of an adjacent channel in the case of the communication system which uses an adjacent channel, or a LOW-IF system, the cut-off frequency needs to be made smaller than the carrier frequency of an adjacent channel.
In the case of the system using the channel in every other one, like PHS (Personal Handy-phone System), unnecessary signals need to be removed by making cut-off frequency smaller than the carrier frequency of the following adjacent channel in which unnecessary signals actually exist.
Because amplifying an unnecessary signal by using the output buffer of a narrow zone is lost, the linear amplification range of an output buffer can be chosen small. By making the linear amplification range small, current consumption can be made few. Naturally, there are many mixer circuits of current output type like the Gilbert multiplication circuit. A current-voltage converter can be used as a buffer amplifier connected to the output terminal of these mixer circuit of current output type. The input impedance of a current-voltage converter is small, and voltage change of the output terminal of a mixer circuit becomes small.
For this reason, there is also an advantage which can avoid the distortion by output voltage change. Because the conventional channel-select filter was designed on the assumption that a voltage input, the conventional frequency converter needed to use the current-voltage converter as a buffer amplifier. And in order not to affect the characteristic of a channel-select filter, the frequency characteristic of this buffer amplifier needed to be the broadband. The current-voltage converter of the frequency converter by this invention has the frequency characteristic of the low-pass passage type cut-off frequency of which being higher than desired signal frequency band and being lower than adjacent channel carrier frequency. For this reason, the large signal of an adjacent channel does not need to be amplified linearly and current consumption can be made small.
Furthermore, this invention offers the frequency converter having the mixer circuit which inputs the radio-frequency input signal modulated for communication of information, and a local oscillation signal, and converts frequency, and the output buffer amplifier comprised from the adjoint circuit of a Sallen-Key circuit, and a current-voltage converter, and characterized by using variable capacity elements as capacity elements in the adjoint circuit. In this invention, an occupancy frequency band can be changed if needed.
Furthermore, this invention offers the frequency converter having the mixer circuit which inputs the radio-frequency input signal modulated for communication of information, and a local oscillation signal, and converts frequency, and the output buffer amplifier comprised from the adjoint circuit of a Sallen-Key circuit, and a current-voltage converter, and characterized by using variable resistance elements as resistance elements in the current-voltage converter. According to this invention, a conversion gain is changeable.
Furthermore, this invention provides the frequency converter having the mixer circuit which inputs the radio-frequency input signal modulated for communication of information, and a local oscillation signal, and converts frequency, and the output buffer amplifier comprised from the adjoint circuit of a Sallen-Key circuit, and a current-voltage converter, and characterized by using variable resistance elements as resistance elements in the adjoint circuit and the current-voltage converter.
Moreover, this invention provides the frequency converter in which, two or more steps of buffer amplifiers are connected to the mixer circuit in the above-mentioned constitution.